1. Field of the Invention
The present invention relates to a digital amplifier apparatus and a method of muting a digital amplifier apparatus. The invention is suitable for application to digital amplifiers (D-class amplifiers) of, for example, Pulse-Width Modulation (PWM) type.
2. Description of the Related Art
Hitherto, with audio apparatuses incorporating a digital amplifier, the user may push the mute button, thus performing a muting operation, while the speaker is generating sound. Then, the speaker must be forcedly stopped, ceasing to generate sound.
At the time the user performs the muting operation, the two PWM outputs of the BTL-connected amplifier circuit are set in the same phase and at a duty ratio of 50% (i.e., “low” state and “high” state being of the same ratio). As a result, the speaker connected to the amplifier circuit stops generating audio outputs.
More specifically, as shown in FIG. 7, a PWM-signal amplifying unit 1 receives a PWM signal P1(+) supplied from a PWM-signal generating circuit (not shown) provided in the stage preceding the unit 1. In the PWM-signal amplifying unit 1, the PWM signal P1(+) is supplied to the gate of a transistor Tr1 that is a Metal Oxide Semiconductor-Field Effect Transistor (MOS-FET). Meanwhile, a PWM signal P1(−) is supplied to the gate of a transistor Tr2 that is a MOS-FET. Therefore, the transistor Tr1 and the transistor Tr2 are alternately turned on and off.
Similarly, in the PWM-signal amplifying unit 1, a PWM signal P2(+) supplied from the PWM-signal generating circuit is applied to the gate of a transistor Tr3 that is a MOS-FET, and a PWM signal P2(−) is supplied to the gate of a transistor Tr4 that is a MOS-FET. Therefore, the transistor Tr3 and the transistor Tr4 are alternately turned on and off.
Thus, in the PWM-signal amplifying unit 1, when the transistor Tr1 and the transistor Tr2 are turned on and off, respectively, and the transistor Tr3 and the transistor Tr4 are turned off and on, respectively, a drive current iA that corresponds to the PWM output OUT1 of the transistors Tr1 and Tr is supplied via a low-pass filter LPF1 to a speaker SP.
In the PWM-signal amplifying unit 1, too, when the transistor Tr3 and the transistor Tr4 are turned on and off, respectively, and the transistor Tr1 and the transistor Tr are turned off and on, respectively, a drive current iB that corresponds to the PWM output OUT2 of the transistors Tr3 and Tr4 is supplied via a low-pass filter LPF2 to the speaker SP.
That is, in the PWM-signal amplifying unit 1, potential differences d1 and d2 develop between the PWM output OUT1 and the PWM output OUT2 as shown in FIGS. 8A and 8B if the PWM output OUT1 and PWM output OUT2 have different duty ratios. The drive currents iA and iB, which correspond to the PWM outputs OUT1 and OUT2, respectively, flow to the speaker SP. As a result, the speaker SP generates sound.
In the PWM-signal amplifying unit 1, no potential difference develops between the PWM output OUT1 and PWM output OUT2 as shown in FIG. 8C if the PWM output OUT1 and PWM output OUT2 have the same duty ratio of 50% and are completely in the same phase. In this case, neither drive current iA nor drive current iB flows to the speaker SP. Hence, the speaker SP stops generating sound. A muting operation is thereby accomplished.
In the PWM-signal amplifying unit 1, the transistors Tr1 and Tr2 are alternately turned on and off and the transistors Tr3 and Tr4 are alternately turned on and off (thus, performing switching), even while the muting operation continues. The waveforms of the PWM outputs OUT1 and OUT2 inevitably have strains such as an overshoot component OS, an undershoot component US, a ringing component LNG and the like, which may cause unnecessary radiation.
That is, the waveforms of the PWM outputs OUT1 and OUT2 generated as the PWM-signal amplifying unit 1 performs the switching are perfectly rectangular as shown in FIG. 9A if dv/dt (voltage change per unit time) is infinitely great. In reality, dv/dt is limited. As shown in FIG. 9B, the outputs will inevitably contain strains such as an overshoot component OS, an undershoot component US, a ringing component LNG and the like, which may cause unnecessary radiation.
In view of this, the switching may not be performed in accomplishing the muting operation. In this method, the PWM outputs OUT1 and OUT2 are forcedly set to the “low” state when the user performs muting operation while the speaker is generating sound.
However, the audio output will become discontinuous as shown in FIG. 1 in the PWM-signal amplifying unit 1, if the PWM outputs OUT1 and OUT2 are abruptly set to the “low” state while the audio output is being generated with a potential difference imposed between them.
An audio-signal generating apparatus and an audio-signal generating method, both designed to reduce pop noise, are known. In the apparatus and method, the widths of pulses in the “high” state are reduced for the PWM outputs OUT1 and OUT2 during the muting operation. (See, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2004-336765)
In the method described above, the on-period over the pulse width is short for the PWM outputs OUT1 and OUT2. Nonetheless, the possibility that a large pop noise is generated is low because the PWM outputs OUT1 and OUT2 are held in the “low” state while the audio output is being generated. However, a small pop noise is inevitably generated.
The present invention has been made in consideration of the foregoing and proposes a digital amplifier apparatus and a method of muting a digital amplifier apparatus, each capable of performing a muting process without generating unnecessary radiation or noise.